1. Field of the Invention
The present invention relates to a rheological additive for coating compositions, especially high solids coating compositions. The present invention also relates to non-aqueous coating compositions containing the rheological additive
2. Description of the Prior Art
Recently the art has sought to reformulate its coating products in an effort to comply with certain government regulations on the emission of organic solvents from surface coating operations. One area of substantial activity has been in the development of high solids solvent-borne coatings, especially high solids baking enamels. These systems have gained a great deal of acceptance in several areas such as coatings for metal furniture and fixtures, metal machinery and equipment, metal appliances, automotive topcoats, can coatings and flat stocks.
To comply with government regulations (e.g., Volatile Organic Content ("VOC") standards), the higher molecular weight resins used in conventional solvent coating systems have been replaced with lower molecular weight resins in high solids coating compositions which have a low solvent content. The conventional coatings, which include higher molecular weight resins, generally have high package viscosities and, therefore, are typically diluted substantially with a solvent prior to application. This reduces the viscosity so that the coating composition may be sprayed. These conventional compositions generally do not experience significant problems with viscosity and sag resistance during baking due to the combined effect of three factors. That is, although an increase in temperature will reduce coating viscosity prior to crosslinking, the evaporation of the solvent and subsequent crosslinking of the resin will increase the viscosity. Thus, the reduction in viscosity due to an increase in temperature is generally offset by an increase in viscosity due to the loss or evaporation of solvent and an increase in viscosity occurs during curing which reduces sagging.
In contrast to the conventional compositions described above, high solids coating compositions are usually formulated at viscosity levels which are suitable for direct application (i.e., without dilution with solvent). This is done by utilizing low molecular weight resins with their associated low viscosities. Unfortunately, the coatings industry has experienced two major rheological related problems associated with the development of acceptable high solids coating systems. Specifically, significant film sagging on a non-horizontal surface will occur during the heat cure cycle and excessive pigment settling will occur during storage. Unlike conventional systems, the lowering of viscosity with heat far outweighs the rise in viscosity that occurs with evaporation of solvent. This results in a substantial amount of sagging prior to the increase in viscosity upon curing. Such sagging adversely affects the appearance of the cured coating. Furthermore, the presence of a rheological additive in high solids coating compositions is required to prevent pigment settling due to their lower package viscosities.
In paint and coating formulations, both film sagging and pigment settling properties have been adjusted by using thixotropic agents. It is generally recognized that to be effective in high solids coating composition systems, a thixotropic agent must provide for the desired rheological properties while avoiding undesired side effects such as excessive viscosity build-up or gloss reduction of the cured paint film. In this regard, there have been several types of thixotropes which have been used to impart anti-sag and anti-settling properties to high solids systems. These thixotropes include fumed silica, precipitated silica, organomodified clays, basic calcium sulfonate gels, cellulose acetate butyrate and microgels.
The foregoing known thixotropes have significant disadvantages. The use of fumed silica and precipitated silica, in order to achieve effective sag control, leads to a loss of gloss and viscosity levels which are too high. Also, fumed silica and precipitated silica must be added to the grind stage of the paint preparation and cannot be used in a post-addition context. Cellulose acetate butyrate leads to undesirably high viscosity levels of the coating system. Furthermore, because they are highly inefficient, it is necessary to use high levels of microgels which is a serious drawback due to their cost. Basic calcium sulfonate gels, such as those available from Lubrizol Corporation and distributed by Ashland Chemical Company under the tradename IRCOGEL 905, are not very efficient in high solids systems.
Another additive which has been used in an attempt to improve the rheology of high solids coatings is described in U.S. Pat. No. 4,526,910. In particular, the additive comprises inorganic microparticles having surfaces that are substantially free of carbon-containing molecules chemically bonded to the inorganic microparticles.
An organic microgel which is prepared by crosslinking an acrylic resin with a multifunctional crosslinking agent is set forth in U.S. Pat. No. 4,290,932. This additive is used for rheology adjustment in acrylic based high solids paints.
Although the use in the art of the above-described inorganic microparticles and organic microgels have shown rheology improvement for a few high solids systems, when applied generally to high solids systems, such as polyester, acrylic or alkyd resin systems, the effect is very marginal, with some of the additives showing incompatibility.
Despite the numerous types of rheological additives known in the art, an ongoing search has been made for new rheological additives for high solids coating compositions as well as conventional organic solvent-based compositions. The present invention is the result of such an investigation.